People Power
Objectives
Students will be able to
• explain the difference between
work and power; and
• convert from one unit of
power to another.
Rationale
This activity allows students to use
their own experience to explain the
difference between work and power.
Materials
• Copies of Calculating Work and
Power by Climbing Stairs,
Student Book, page 10
• Bathroom scales (optional)
• Clipboards (optional)
• Rulers (yardsticks or meter
sticks are optional)
• Stopwatch, digital watch, or
watch with second hand
• Light bulb, preferably a 100-watt
incandescent bulb (optional)
Background
When people say the words energy, work, or
power in everyday conversations, listeners
usually have little trouble understanding
what these words mean. For example,
one teacher might say to another, “I put a
lot of work into my lesson plan last night,
and it paid off. Today’s class went really
well.” The other teacher would understand
immediately what the first is talking about.
But what are the scientific definitions
of energy, work, and power? Energy is
often defined as the ability to do work.
In turn, work has a specific definition in
physics—it is equal to the force needed
to move an object multiplied by the
distance it is moved. Although the teacher
planning the next day’s lesson during the
evening may say that he is doing work,
by definition, work is done only when the
teacher actually moves something, such as
moving his pen to write his lesson plan.
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People Power
To determine how much work a person
does to move himself or herself, we need
to measure the force exerted and the
distance traveled. An easy way to do this
is to have a student climb a flight of stairs.
To climb the stairs, the student must do
work to overcome gravity. The work done is
equal to her weight, which is the force she
must exert to overcome gravity, multiplied
by the height of the staircase. For example,
the work done by a student weighing 550
Newtons (N) and climbing a 3-meter-high
staircase is:
Work = Force x Distance
= student’s weight x height of staircase
= 550 Newtons x 3 meters
= 1,650 joules
NOTE: The Newton is the metric unit for
weight, which is an object’s mass (kg) times
the acceleration due to gravity (9.8 m/s2.
550 Newtons is equal to about 125 pounds
(1 lb. = .45 kg). To figure out your weight in
Newtons using a bathroom scale, multiply
the weight in pounds by .45 kg and then by
the acceleration due to gravity (9.8 m/s2).
The joule (j) is the unit used to measure
work. One joule of work requires that one
Newton of force push an object a distance
of one meter
work = force x distance
1 joule = 1 Newton x 1 meter.
The horizontal distance she traveled when
going up the stairs is not counted since
this distance is not in a direction that
overcomes gravity.
Would the work done by a student slowly
walking up the staircase be different than
if she ran up the same staircase? No,
because the definition of work does not
take into account the time needed to climb
the stairs (work equals force multiplied
by distance). Yet the two situations are
different. When the student runs up the
stairs, she does the same amount of work
Summary: Students discover
the difference between work and
power by climbing stairs slowly and
quickly and also learn to convert
from one unit of power to another.
Grade Level:
5–8 (9–12)
Subject Areas:
Language Arts, Mathematics
(Arithmetic), Science (Physical),
Setting:
Classroom, Staircase
Time:
Preparation: 15 to 30 minutes
Activity: two 50-minute
class periods
Vocabulary:
Electrical
generator, Horsepower, Joule,
Newton, Power, Turbine, Watt
Major Concept Area:
• Definition of energy
Standards Addressed:
Common Core ELA: RLST.6-12.4
Common Core Math: 2.MD.1,
3.NBT.3, 3.OA.7, 4.MD.1, 5.MD.1,
5.NBT.7, 5.NF.3
NGSS: HS-PS3-1
DCI: PS3.A: Definitions of Energy,
PS3.B: Conservation of Energy and
Energy Transfer
CCC: Systems and System Models
Getting Ready:
Tell students to wear athletic shoes
on the day the activity will take
place.
Resources:
For a list of additional
resources related to this
activity, visit the KEEP website
at keepprogram.org and click
on Curriculum & Resources
© 2016 KEEP
l theme I: we need energy l KEEP Energy Education Activity Guide
Related KEEP Activities:
Have students explore where they
get their energy to conduct this
activity through “Roasted Peanuts.”
Extend this activity by having
students do conversion factor
calculations using “Energy and
Power Conversion Factors” in the
Appendix. Additional information
about generating electricity is
found in “Diminishing Returns.”
in less time than when she walks up the
stairs. In other words, she works faster.
The term that expresses this difference is
power, which is defined as the work done
per unit of time, or the rate of doing work.
In terms of a formula
Power = work done
time
for stair climbing
Power =
weight (Newtons) × height of stairs (meters)
time (seconds)
The power output of the student walking
up the stairs in 10 seconds is
Power = work done
time
= 550 N × 3 m
10 seconds
= 165 watts
The power output of the same student
running up the stairs in two seconds is
Power = 550 N × 3 m
2 seconds
= 825 watts
One watt is equal to one joule of work
done per second. Watts are a unit of power
often associated with electrical equipment
like light bulbs, hair dryers, and stereo
amplifiers. Watts also describes the power
output of engines and students running up
stairs. The watt is a small unit of power.
Therefore, multiples of the watt, such as
the kilowatt (1,000 watts) or the megawatt
(one million watts), are often used.
NOTE: In the United States, power for
large motors such as in lawn mowers and
automobiles is measured in an English unit
called horsepower. One horsepower equals
about three-quarters of a kilowatt. (See
Watt’s a Horsepower? for the origin of the
unit of horsepower.)
Remember, power is the rate at which work
is done. See the Power Data Table for
sample power outputs.
In the stair climbing exercise, the student
running up the stairs does 825 joules
of work every second. At the end of two
seconds, the time it takes the student to
reach the top of the stairs, she will have
done 1,650 joules of work.
Saying that a student expended four-fifths
of a kilowatt of power running up the stairs
sounds impressive. This is the same
amount of power produced by more than
eight 100-watt light bulbs. However, if the
student’s power output from running up the
stairs could somehow be converted into
electrical power, the bulbs would only stay
on for two seconds.
Furthermore, a student cannot maintain a
power output of four-fifths of a kilowatt by
running up a staircase higher than 3 meters
for more than a few seconds without
growing tired and slowing down.
Running up stairs is not a practical way to
produce electricity. Is there another way to
convert a student’s mechanical power into
electrical power for light bulbs? The student
could ride a stationary bicycle whose rear
wheel is connected to a small electrical
generator wired in a circuit to a light bulb.
When the student pedals the bicycle, she
generates electricity and lights the bulb.
In fact, generating electricity using a power
plant is similar to generating electricity by
riding a stationary bicycle. The boiler in a
KEEP Energy Education Activity Guide
power plant converts the chemical energy in
fossil fuels into the kinetic energy of steam
by burning fossil fuels to boil water. This
process is like a student’s body converting
the chemical energy from food into the
mechanical energy of moving muscles. Like
the rear wheel of the stationary bicycle,
the turbine in a power plant is connected
to an electrical generator. Heated steam
pushing on the turbine blades spins the
turbine, much like a student pushing on
bicycle pedals spins the rear bicycle wheel.
The mechanical energy of the spinning
turbine or the spinning bicycle wheel is
then converted into electrical energy by
the generator. The electricity produced by
the power plant generator travels through
transmission lines to the lights in our
homes. Similarly, the electricity produced
by the generator connected to the bicycle
wheel travels through the wires of a circuit
to light a light bulb.
Finally, what about the teacher mentioned
earlier who put a lot of work into his lesson
plan? Does his brain’s actual power output
increase with extra mental effort? Studies
that have measured the brain’s power
output showed that the difference between
thinking hard and not doing so was about
four watts—little more than the power
output of a small candle burning to the end.
Procedure
Orientation
Ask students if a person does more work
walking up a flight of stairs or running up
the same flight of stairs.
Steps
1. Have students find a staircase,
measure its height, and time how long
it takes to walk up and run up the
staircase. Students may try one of the
following four approaches. NOTE: These
approaches should be considered with
respect to students who may be selfconscious about their weight or physical
abilities.
l theme I: we need energy l People Power
2
• Have students locate, measure, and
climb a flight of stairs on their own
time and bring their results to class.
• Have one or two students walk
up and run up the stairs while
the rest of the class waits.
• Provide students with a set
amount of time with which to find
a staircase in the school and
conduct the measurements.
• Walk up and run up a staircase yourself
and provide students with the data.
NOTE: The reason students are climbing
stairs to measure the work they do is
because their weight, which is equal to
the force they need to overcome gravity,
can easily be measured. Measuring
the force they exert with their feet
while walking across a floor is much
more difficult (see Background).
2. Hand out and have students
complete Parts I, II, and III of the
activity sheet, Calculating Work
and Power by Climbing Stairs.
3. Discuss how students felt after
climbing the stairs slowly compared
to climbing them rapidly. If they did
the same amount of work, why did
they feel differently? They should note
that the amount of time they used to
climb the stairs was different. Explain
to students that because it took less
time to do work when running up the
stairs, they expended more power.
4. Have students complete Part IV of the
activity sheet. Discuss the different
units of measurement for power. Relate
them to different units of measuring
length, such as meters and yards. Of
the three units used to measure power,
which have they heard of before?
5. Show students a 100-watt bulb. What do
they think “100-watts” listed on the top
of the bulb means? Explain that it refers
to the power output of the bulb. A light
bulb with a power output of 100 watts
converts 100 joules of electrical energy
into light and heat energy every second.
6. Ask where the energy to light the bulb
comes from. Students may mention
putting the bulb into a socket, flipping
a switch, providing it with electricity,
etc. Challenge students to explain
how we get electricity. Briefly explain
that one way to get electricity is to
generate it in a power plant using
fossil fuels (see Background).
7. Ask students if the power they expended
by climbing the stairs could somehow
be converted into electrical power.
If so, could they produce enough
electricity to light a light bulb?
8. Have students complete Part V of
the activity sheet to determine how
their power output from climbing
the stairs relates to the power
output of a 100-watt light bulb.
9. Ask students if it is possible to convert
the power produced by a person into
electrical power. Describe how this
conversion can be done with a person
riding a stationary bicycle with its
rear wheel connected to an electrical
generator. You may also compare
generating electricity using a bicycle
with the way a power plant generates
electricity (see Background).
Closure
Have students summarize their stairclimbing activity using the words work
and power. Discuss what happened to the
energy they used to climb the stairs.
Assessment
Formative
• Are students’ activity sheets completed
thoroughly and accurately?
• Are students able to explain the
difference between work and power?
• Are students able to convert from
one unit of power to another?
• Can students relate the power output
of climbing stairs to other examples
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like engines and light bulbs?
Summative
• Have students give other personal
examples that illustrate the difference
between work and power.
• Have students calculate how much
time it would take a person to climb
stairs, if they know the height of the
stairs, the weight of the person, and
the amount of power produced.
Discuss the origin of the horsepower
unit by having students read Watt’s a
Horsepower? After reading this, have
students define a unit of their own such
as the “studentpower,” “humanpower,”
or “childpower,” and then convert the
horsepower figures from their Calculating
Work and Power by Climbing Stairs activity
sheets into the unit they have defined.
Example: The power output of a 240pound (109 kg) football player running up
a 25-foot (7.6 meters) high set of stairs
in a stadium is equal to 1.5 horsepower
(1,119 watts). How long did it take him to
run up the stairs? (Answer: 7.2 seconds)
Extensions
Have the class do this activity using
metric units for height, weight, work, and
power. The class could also convert the
units of energy used in this activity to
other units such as Btus and calories.
Challenge students to come up with
ways to convert the mechanical energy
of a person to other forms of energy.
Power Conversion Chart
watt
(W)
kilowatt
(kW)
megawatt
(MW)
horsepower
(hp)
foot-pounds/second
(ft-lbs/sec)
1
watt
=
1
0.001 kW
0.000001 MW
0.00134 hp
0.738 ft-lbs/sec
1
kilowatt
=
1,000 W
1
0.001 MW
1.34 hp
738 ft-lbs/sec
1
megawatt
=
1,000,000 W
1,000 kW
1
1,340 hp
738,000 ft-lbs/sec
1 horsepower =
746 W
0.746 kW
0.000746 MW
1
550 ft-lbs/sec
1
foot-pound/
second
=
1.36 W
0.00136 kW
0.00000136 MW
0.00182 hp
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KEEP Energy Education Activity Guide
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Watt’s a Horsepower?
The origin of the term horsepower seems obvious: it is the
power output of a horse doing work of some kind. But what
exactly does this mean? How did the power of a horse come
to be defined as precisely 550 foot-pounds per second?
When steam engines first appeared in England at the turn of
the eighteenth century, they were used to pump water from
mines, a task that horse-driven pumps had previously done.
Soon, steam engines were rated in terms of the number
of horses they could replace, or their “horsepower.” For
example, a steam engine might be rated at four horsepower,
meaning that it could replace four horses. Rating steam
engines this way gave people a general idea of a steam
engine’s power, but it was not a precise definition. How much
power could horses actually produce? Were they big horses
or small ones? What if the horses got tired after a while?
Attempting to answer these questions, a number of steam
engine inventors and developers defined horsepower on
their own. Some of these definitions even had specific
values. For a time, however, no one could agree on a
single definition for this unit, so few people took the
horsepower rating of steam engines seriously.
By the late eighteenth century, James Watt (1736–1819)
had made major improvements to the steam engine (he did
not invent it). Because of his work, steam engines became
more efficient and economical to use. Watt eventually
became a successful businessman and by 1800, he and
his partners had produced over 500 steam engines and his
improvements were readily adapted by other manufacturers.
Watt understood that having different definitions of
horsepower was confusing. How could customers compare
steam engines made by different manufacturers, if different
horsepower definitions were used? To end the confusion,
Watt sought a definition of horsepower that everyone
could agree with. He made a series of measurements to
see how much weight an average horse could lift steadily.
From this, Watt arrived at a figure of 550 foot‑pounds
per second. This value was eventually adopted and is
now the accepted definition of one horsepower.
However, there is nothing special about this particular
definition. A horsepower could easily have been defined as
500 foot-pounds per second. What must be remembered is
that definitions of units are often chosen for convenience
and are not necessarily related to the laws of science.
People agree to use certain units because the definitions
are sensible and because those who define the units are
often in a position of influence in society. In the case of
Watt, the success of his steam engines and his prominence
as a scientist and businessman played as much a role
in the adoption of his horsepower definition as did his
measurements of a horse’s ability to lift weights.
Not only did Watt define horsepower, his fame eventually
led the scientific community to name a metric unit of power
after him. Today, one watt is defined as one joule of energy
per second, and one horsepower equals 746 watts.
Steam Engine
Photo credit: Baptist | Shutterstock
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Power Data Table
Horsepower
Watts
0.00094
0.7
0.004
3
0.034 to 0.402
0.012 to 0.044
0.005 to 0.020
25 to 300
9 to 33
4 to 15
0.25 to 2
187 to 1,500
1.74
1,300
1.34 to 2
900 to 1,500
0.94
3
700
2,240 (2.24 kW)
10.7 to 21.4
134
8 to 16 kW
100 kW
66 to 640
89
185
49 to 477 kW
66 kW
138 kW
The Columbia power plant in Portage, Wis.
1.3 million
1,000 MW
A moon rocket during launch
(Apollo 11 on a Saturn C 5 rocket)
3.5 million
2,600 MW
The sun
5.1 x 1023
3.8 x 1026
A hummingbird in flight
A small candle burning to the end
Typical light bulbs used by homeowners
Incandescent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compact Fluorescent (CFL). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Light Emitting Diode (LED) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A gasoline-powered lawn mower
An adult human running a 100-meter dash in 10 seconds
A hair dryer
A horse
Average (doing field work). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Maximum (pulling a weight equal to .35 percent of horse’s body weight)
Watt’s steam engines
Typical range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Largest engines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Automobiles
Typical 2017 models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2017 smart fortwo prime. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2017 Honda Accord. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adapted from “Table A2: Power Levels of Ephemeral Phenomena,” p. 320 in Smil, Vaclav. General Energetics: Energy in the Biosphere and Civilization.
New York: John Wiley and Sons, 1991. Used with permission. All rights reserved.
KEEP Energy Education Activity Guide
l People Power
6
Answer Key: Calculating Work &
Power by Climbing Stairs
NOTE: The answer key uses a weight of 125 pounds, a staircase height of 3 meters, and time data (10 seconds and 2
seconds) from the Background to show how the calculations are done. Student answers will be different due to the use of
different weight, height, and time data.
Introduction
In this activity, you will find out if a person does more work walking up a flight of stairs or running up the same flight of stairs
by having you or someone else actually try this. You will also learn what scientists mean by the words work and power.
Part I
1. Find a staircase that is not used by a lot of people. Using a ruler, record the height of one step in centimeters in Table 1.
2. Count the number of steps that you or your classmate will be climbing and record your answer in Table 1.
3. Calculate the total height of the staircase in centimeters.
4. Calculate the total height of the staircase in meters.
Table 1
1. Height of one step (cm)
18 cm
2. Number of steps climbed
17 steps
3. T otal height of staircase (cm)
= height of one step multiplied by number of steps climbed
17 steps x 18 cm per step
= 306 cm
4. T otal height of staircase (m)
= total height of staircase (cm) / 100
306 cm / 100
= 3.06 meters
= 3 meters (rounded answer)
Part II
5. Next, decide who will be climbing the stairs and who will be recording the time of the stair climber. Write the name
and the weight of the stair climber in the blank spaces below. If the stair climber does not know their weight, either
use a scale and measure it, estimate the stair climber’s weight, or use a standard weight of 100 pounds.
Name of stair climber Jane Smith
Weight of stair climber 125
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People Power
pounds
l KEEP Energy Education Activity Guide
6.To figure out weight in Newtons, multiply pounds by 0.45 kg and then by the acceleration due to gravity
9.8 meters / sec2 = ______N.
125 pounds x 0.45 kg per pound x 9.8 meters/sec2
= 551 kg-m / sec2
= 550 Newtons (rounded answer)
7. Have the stair climber climb the stairs slowly and steadily. Record the time it takes in seconds in Table 2 under the column
labeled “Slowly.” You may want to practice this a few times to get an accurate time.
8. Now have the same stair climber climb the stairs rapidly. This does not mean that the stair climber has to risk getting
hurt by climbing the stairs as fast as possible. Record the time it takes in seconds in Table 2 under the column labeled
“Rapidly.” You may want to practice this a few times to get an accurate time.
NOTE: It is important that the same stair climber climb the stairs slowly and rapidly because the same weight must be used to
compare the work done while climbing the stairs for both cases.
Table 2
Climbing Stairs
Slowly
Rapidly
Time Required
10 seconds
2 seconds
Part III
9. Calculate the work done by the stair climber in climbing the stairs slowly and rapidly using units of Joules. Record your
answers in Table 3 below.
The formula for work done in joules is:
Formula:
= weight of stair climber (Newtons) x height of staircase (m)
Table 3
Climbing Stairs
Work done (joules
Slowly
Rapidly
550 Newtons x 3 meters
550 Newtons x 3 meters
= 1,650 joules
= 1,650 joules
KEEP Energy Education Activity Guide
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10. Was the work done by climbing the stairs slowly the same as or different from climbing the stairs rapidly? Does your
answer surprise you? Record your responses in the blank space below.
The same work was done when climbing the stairs slowly compared to climbing the stairs rapidly (= 1,650 Joules). This may
seem surprising at first. However the formula for calculating the work done does not include the time it takes to climb the
stairs – it only includes the weight of the student and the height of the stairs.
Part IV
11. Calculate the power output of climbing the stairs slowly and rapidly in watts. Record your answers in Table 4 below.
The formula for calculating the power output in watts is:
Formula:
work done climbing stairs (joules
Time required to climb stairs (seconds)
Table 4
Climbing Stairs
Slowly
Power output (watts)
Rapidly
1,650 joules / 10 seconds
1,650 joules / 2 seconds
= 165 watts
= 825 watts
Part V
12. Suppose the power output of a person climbing stairs could somehow be directly converted into electrical power. How
many 100-watt light bulbs could a person light by climbing the stairs slowly and rapidly? Record your answers in Table 5
below.
The formula for calculating the number of 100-watt light bulbs a student could light is:
Formula:
Power output of student (watts)
100 watts per bulb
Table 5
Number of 100 watt light bulbs a
person could light by climbing stairs
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People Power
Slowly
Rapidly
165 watts / 100 watts per bulb
825 watts / 100 watts per bulb
= 1.65 light bulbs
= 8.25 light bulbs
l KEEP Energy Education Activity Guide
Calculating Work & Power
by Climbing Stairs
Introduction
In this activity, you will find out if a person does more work walking up a flight of stairs or running up the
same flight of stairs by having you or someone else actually try this. You will also learn what scientists
mean by the words work and power.
Part I
1. Find a staircase that is not used by a lot of people. Using a ruler, record the height of one step in
centimeters in Table 1.
2. Count the number of steps that you or your classmate will be climbing and record your answer in
Table 1.
3. Calculate the total height of the staircase in centimeters.
4. Calculate the total height of the staircase in meters.
Table 1
1. Height of one step (cm)
2. Number of steps climbed
3. T otal height of staircase (cm)
= height of one step multiplied by number of steps climbed
4. T otal height of staircase (m)
= total height of staircase (cm) / 100
Part II
5. Next, decide who will be climbing the stairs and who will be recording the time of the stair climber.
Write the name and the weight of the stair climber in the blank spaces below. If the stair climber
does not know their weight, either use a scale and measure it, estimate the stair climber’s weight, or
use a standard weight of 100 pounds.
Name of stair climber_______________________________________________________________
Weight of stair climber ________________________________________________________ pounds
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6. To figure out weight in Newtons, multiply pounds by 0.45 kg and then by the acceleration due to
gravity 9.8 meters / sec2 = ______N.
7. Have the stair climber climb the stairs slowly and steadily. Record the time it takes in seconds in
Table 2 under the column labeled “Slowly.” You may want to practice this a few times to get an
accurate time.
8. Now have the same stair climber climb the stairs rapidly. This does not mean that the stair climber
has to risk getting hurt by climbing the stairs as fast as possible. Record the time it takes in seconds
in Table 2 under the column labeled “Rapidly.” You may want to practice this a few times to get an
accurate time.
NOTE: It is important that the same stair climber climb the stairs slowly and rapidly because the same
weight must be used to compare the work done while climbing the stairs for both cases.
Table 2
Climbing Stairs
Slowly
Rapidly
Time Required
Part III
9. Calculate the work done by the stair climber in climbing the stairs slowly and rapidly using units of
Joules. Record your answers in Table 3 below.
The formula for work done in joules is:
Formula:
= weight of stair climber (Newtons) x height of staircase (m)
Table 3
Climbing Stairs
Slowly
Rapidly
Work done (joules
KEEP Energy Education Activity Guide - Student Book
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10. Was the work done by climbing the stairs slowly the same as or different from climbing the stairs
rapidly? Does your answer surprise you? Record your responses in the blank space below.
Part IV
11. Calculate the power output of climbing the stairs slowly and rapidly in watts. Record your answers in
Table 4 below.
The formula for calculating the power output in watts is:
Formula: work done climbing stairs (joules
Time required to climb stairs (seconds)
Table 4
Climbing Stairs
Slowly
Rapidly
Power output (watts)
Part V
12. Suppose the power output of a person climbing stairs could somehow be directly converted into
electrical power. How many 100-watt light bulbs could a person light by climbing the stairs slowly and
rapidly? Record your answers in Table 5 below.
The formula for calculating the number of 100-watt light bulbs a student could light is:
Formula: Power output of student (watts)
100 watts per bulb
Table 5
Slowly
Rapidly
Number of 100 watt light bulbs
a person could light by climbing
stairs
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